Novel chemical base for fuel cell engine heat exchange coolant/antifreeze

ABSTRACT

A nontoxic fuel cell engine coolant which has an electrical resistivity of greater than 250 kohm-cm, a boiling point of greater than 90° C., a freezing point of less than −40° C., a thermal conductivity of greater than 0.4 W/m-k, a viscosity of less than 1 cPs at 80° C., a viscosity of less than 6 cPs at 0° C., a heat capacity of greater than 3 kJ/kg-K, and which is compatible with current cooling system materials.

FIELD OF THE INVENTION

This invention relates to a novel technology for use in cooling systemsfor fuel cell powered vehicles and/or equipment. In order to remove theheat that is generated in fuel cell systems, 1,3-propane diol is used asthe chemical base for the heat exchange fluid.

BACKGROUND OF THE INVENTION

It has been suggested that fuel cell technology can be used to generateelectricity in sufficient volume to be applicable in the driving ofelectric motors for passenger vehicles, standby power generation, andother applications. A fuel cell is a device that converts chemicalenergy of a fuel directly into electricity and they are intrinsicallymore efficient than most other energy generation devices, such asinternal combustion engines. In principle, a fuel cell operates somewhatlike a battery. Unlike a battery, a fuel cell does not run down orrequire recharging. It will produce energy in the form of electricityand heat as long as fuel is supplied. The most common type of fuel cellconsists of two electrodes sandwiched around an electrolyte. Oxygenpasses over one electrode and hydrogen over the other, generatingelectricity, water, and heat.

The fact that heat is generated by the fuel cell requires the presencein the automobile or other system of a cooling system which can besimilar to those used presently in internal combustion engines.Typically, such a system includes a circulating pump, plumbing that mayinclude aluminum, brass, copper, lead-tin solder, stainless steel,plastic or rubber materials, and a heat exchanger (radiator) typicallyconstructed of aluminum or copper/brass.

The heat exchange fluid (coolant) is obviously just as important in afuel cell system as it is in internal combustion engines. Many of therequirements of a heat exchange fluid for internal combustion enginesare also required for fuel cell engines. However, there are someadditional requirements. For instance, fuel cell vehicles generate adirect current of 400 volts. The coolant, which flows around thealuminum components of the fuel cell, must be nonconductive to protectboth the cell itself from shorting out and to prevent electrical hazardto humans operating or servicing the system.

The first fuel cell was built in 1839 by Sir William Grove, a Welshjudge and gentleman scientist. The “Grove cell” used a platinumelectrode immersed in nitric acid and a zinc electrode in zinc sulfateto generate about 12 amps of current at about 1.8 volts. There wereother developments in fuel cell technology over the years but seriousinterest in the fuel cell as a practical generator of electricity didnot begin until the 1960's, when the U.S. Space Program chose fuel celltechnology over nuclear power and solar energy. This technology,developed by Francis Thomas Bacon, used nickel gauze electrodes andoperated under pressures as high as 300 psi.

SUMMARY OF THE INVENTION

A nontoxic fuel cell engine coolant which has an electrical resistivityof greater than 250 kohm-cm, a boiling point of greater than 90° C.,optionally, a freezing point of less than −40° C., a thermalconductivity of greater than 0.4 W/m-k, a viscosity of less than 1 cPsat 80° C., a viscosity of less than 6 cPs at 0° C., a heat capacity ofgreater than 3 kJ/kg-K, and which is compatible with current coolingsystem materials. The coolant may contain from 1 to 100, preferably 40to 85 and most preferably 55 to 85, volume percent PDO and most or allof the remaining balance is water.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the aqueous solution freeze point characteristics ofthe 1,3-propanediol and GM 6043 inhibition chemistry (EG).

FIG. 2 is a plot of the freeze behavior of aqueous 1,3-propanediolantifreeze.

DETAILED DESCRIPTION OF THE INVENTION

As previously stated, the purpose of a fuel cell is to produce anelectrical current that can be directed outside the cell to do work,such as powering an electric motor. Because of the way electricitybehaves, this current returns to the fuel cell, completing an electricalcircuit. The chemical reactions that produce this current are the key tohow a fuel cell works. There are several kinds of fuel cells whichoperate somewhat differently but in general terms, hydrogen atoms entera fuel cell at the anode where a chemical reaction strips them of theirelectrons. The hydrogen atoms are now “ionized” and carry a positiveelectrical charge. The negatively charged electrons provide the currentthrough wires to do work.

Oxygen enters the fuel cell at the cathode and it there combines withelectrons returning from the electrical circuit and hydrogen ions thathave traveled through the electrolyte from the anode. In some fuel cellsthe oxygen picks up electrons and then travels through the electrolyteto the anode where it combines with hydrogen ions. This chemicalreaction generates a significant amount of heat energy which must beremoved from the fuel cell in order for it to continue to operateproperly.

A number of objectives have been identified for coolants for fuel cellvehicles. First, since fuel cell vehicles generate a direct current of400 volts, the coolant which flows around the aluminum components of thefuel cell must be nonconductive to protect the cell from shorting outand to prevent electrical hazards. Other physical property objectivesfor fuel cell coolants are set out in the table below: TABLE 1Electrical Resistivity >250 kOhm-cm Boiling point >90° C. Freezing point<−40° C. Thermal Conductivity >0.4 W/m-k Viscosity <1 cPs @ 80° C. < 6cPs @ 0° C. Heat Capacity >3 kJ/kg-K Durability >5,000 hours ofoperation/3 years total time Material compatibility: Compatible withcurrent cooling system materials Toxicity Classified as non-toxic fortransportation

1,3-propanediol (PDO), which is manufactured by Shell Chemical Company,is generally made as described in U.S. Pat. No. 5,304,691 and the artdescribed therein. This is a process for making PDO and HPA(3-hydroxypropanal, a 3-hydroxyaldehyde). In this particular patent, PDOand HPA are made by intimately contacting an oxirane (ethylene oxide,hereinafter ‘EO’), a ditertiary phosphine-modified cobalt carbonylcatalyst, a ruthenium catalyst promoter, and syngas (carbon monoxide andhydrogen) in an inert reaction solvent at hydroformylation reactionconditions. A PDO yield of up to 86-87 mole % is reported, using acatalyst comprising cobalt ligated with1,2-bis(9-phosphabicyclononyl)ethane as bidentate ligand, and eithertriruthenium(0) dodecarbonyl or bis[ruthenium tricarbonyl dichloride] ascocatalyst. Other methods of making PDO are known.

Inhibited with the GM 6043 chemistry, the 1,3-propanediol performedsomewhat better than EG in modified ASTM-type tests. FIG. 1 illustratesthe aqueous solution freeze point characteristics of the 1,3-propanedioland GM 6043 (EG). There is a slight compromise of freeze protection asdetermined by the ASTM D1177 test method, but the 1,3-propanediol wassoft and slushy at the reported freeze point. This could be anindication that actual protection against hard, damaging freezing isactually better, approaching the effective protection point of theEG-based product. We also performed the D1177 test with 55% and 60%1,3-propanediol in water, and found that the 55% concentrated productoffered protection equivalent to 50% EG, per the test method. Freezeprotection continued to improve at 60% 1,3-propanediol. We feel that theantifreeze properties of the chemistry are acceptable. Indeed a 50%solution would provide adequate protection against freezing in mostgeographies. TC in FIG. 1 is an internal designation for the PDO aqueoussolutions at 50, 55, and 60 volume percent PDO.

FIG. 2 shows the freeze behavior of PDO/water solutions. It can be seenthat formulations may be made with freeze points significantly lowerthan −40° C.

It may be desirable to include an effective amount of an antifoamingcomposition in the antifreeze/coolant composition. Such components arewell known. Polyglycol-type antifoaming agents can be used.

PDO coolants in fuel cell vehicles will have an electrical resistivityof greater than 250 kOhm-cm, a boiling point of greater than 90° C.,usually a freezing point of less than −40° C., a thermal conductivity ofgreater than 0.4 W/m-k, a viscosity of less than 1 cPs at 80° C. andless than 6 cPs at 0° C., a heat capacity of greater than 3 kJ/kg-K, adesired durability of greater than 5000 hours of operation (three yearstotal time), material compatibility—will not corrode or erode currentautomotive cooling system materials, have a toxicity classified asnon-toxic for transportation, and will be cost competitive with currentautomotive coolants.

The PDO formulations give intrinsically better protection againstcavitation than EG or PG.

It is our theory that some or all of these advantages are based upon therelative chelation ability of PDO versus EO and PO. The latter arereadily able to chelate the ions. The chelate with EO and PO will be afive-membered ring which is relatively easy to form. PDO cannot chelatethe ions as well because it forms a six-membered ring and this is moredifficult.

EXAMPLES

Two chemistries were used in the following experiments. These are1,3-propane diol (anhydrous) and 1,3-propane diol (50 to 85 percentvolume percent aqueous solution).

Example 1

At the beginning, we believed that the classical corrosion andperformance testing regimen as described in ASTM literature (2001 AnnualBook of ASTM Standards, Volume 15. 05) provides an accepted method toevaluate and compare the corrosive properties of coolants to the metalscustomarily used in vehicle coolant systems. The new variable for fuelcells is the 400 volt (Direct Current) electric field and the issuesthat such a field presents to the coolant. Ionic inhibitors aredisqualified. The above coolants, running in the maximum resistancestate with no inhibitors, were reviewed.

We believed that the following tests would accurately predict the abovecoolants' abilities to perform in a heat exchange system, in terms ofcorrosion protection, and physical and chemical properties. Since thesenew coolants had not been through this regimen of testing before, therewas no experience or normal performance against which the tests could becompared for reasonableness. Therefore, each of the tests was controlledagainst 50 volume percent aqueous inhibited ethylene glycol.

The classical coolant development approach involves analyzing the fluidfor physical and chemical properties. Once the properties areestablished, performance objectives are determined and the prototypesevaluated. These tests may be modified to better evaluate theperformance of a coolant in its intended operating environment. Examplesof modifications may include variations in the pressure, temperature,electric fuel environment, and duration of the tests. The data then willbegin to serve to establish comparative and baseline data for theprototype new coolants. These tests will include fundamental properties,such as pH value and specific gravity, physical properties, andcoolant-specific parameters including foaming tendency and reservealkalinity. We believed that this data would direct the research towardsthe most appropriate coolants. The results are shown in Table 2. TABLE 2Physical and Chemical Properties Comparative Current Specification TestNumber & Description Value Comments ASTM D-1122 Relative Density1.110-1.145 The relative An experiment to determine the property ofrelative density of the density. This information is new coolant willused later in verifying the quality of be different commercializedproducts produced at than EG or PG blending facilities, and also hasvalue and will also to estimate contamination levels. depend on theconcentration of PDO and water. ASTM D-1177 Freeze Point <−40° C.Choosing an This experiment overcomes the soft appropriate ‘slushy’freeze characteristic that makes solution can determining the freezingpoint of some satisfy this fluids difficult. It produces a graph ofrequirement. cooling behavior from which a consistent and meaningfulfreeze point can be determined. ASTM D-1120 Boiling Point >90° C. Theboiling This is a boiling point method consistent point of the new withstandard methods used to determine coolant will be the boiling points ofmost fluids. different than EG or PG and will also depend on theconcentration of PDO and water ASTM D-1882 Auto Finish no effect Noproblem The coolant is likely to be spilled on an expected. auto finish.Therefore, it has always been a requirement that the coolant has noeffect on the cars′ finish, and this test was developed to evaluate thatproperty. ASTM D-1119 Ash Content <5.0% max. Since this High levels ofdissolved solids are coolant will be associated with premature waterpump wear very low in and other durability issues. Completelyinhibitors, this evaporating the liquid and calculating specificationthe weight of the remaining dry material may need to be determines ashcontent. further reduced to prevent conductivity problems. ASTM D-1287pH: 7.5 to Experimentation The H⁺ ion concentration is reported as a11.0 will likely pH value. This value is determined from result in a aninstrument reading. The pH value has tighter spec for to be appropriatefor the inhibitor PDO than is used technology in use. today for EG andPG coolants. ASTM D-1123 Water mass percent 5.0% max. Applicable toWater content on non-aqueous coolants is the PDO before determined bythe Karl Fischer method. blending. ASTM D-1121 Reserve Alkalinity Thisproperty may In many inhibition technologies, the be obsolete, ordurability of the coolant is related to may have QC its ability toneutralize weak acids value. formed as the base and/or inhibitorsdegrade. This titration evaluates that property. ASTM D-1881 FoamingTendencies Break: 5 sec. The new coolant Foaming is an undesirableproperty Volume: 150 ml should meet this associated with negativeperformance. requirement. This method creates a measurable volume, andalso the time required to dissipate the foam. Electrical Conductivitymohs <50 Experimental data Test method: a calibrated laboratory to beused in bench conductivity meter is employed to developing a testmeasure the conductivity of the and performance coolant. Theconductivity probe is specification. placed into the fluid, and thedigital reading on the conductivity meter is observed. Viscosity (cPs)ASTM D-445 <1@80° C. Comparable to EG <6@0° C. coolant. ThermalConductivity W/m-K from >0.4 Comparable to EG literature coolant. HeatCapacity (kJ/kg-K) from literature >3 Comparable to EG coolant.Durability by extended duration tests >5 years PDO promises excellentstability. Effect on Elastomers: <10% Δ By Cummins Method 14292Dimension Each Silicon Seals, Viton, Bunan (Nitrile), Teflon, Neoprene,Rubber, Nylon Toxicity LD₅₀ data and review of MSDS Non toxic for PDOoffers low transportation toxicity. ASTM D2809 Water Pump Test, repeated≧8 each time PDO has performed three times better than EG in the seriesof tests. See Table 3 below. ASTM D-4340 Corrosion of Aluminum Heat <1.0mg/cm²/ PDO has performed Rejecting Surfaces week at less than 10% ofthe allowed loss. Extended aging evaluation in D-4340 Rig <1.0 mg/cm²/PDO degraded less @ 150° C. for 60 Days sampled @ 10 day week < 2 pH interms of pH intervals. units value and in the Aluminum weight loss <20%formation of Δ pH <2,000 ppm oxidation by- Oxidation products (i.e. COOHm < 1 products in the anions) presence of two Oxidation trend (slope offully formulated regression) coolant inhibition packages. See Table 4below. ASTM D-1384 Corrosion in Glassware Maximum Weight Test passed.(Higher Performance Loss, mg Specification) Copper 5 Lead Solder 10Brass 5 Steel 5 Cast Iron 5 Cast Aluminum 10 Aged Coolant Corrosion(ASTM D-1384 Maximum Weight extended) in Glassware @ 150° C. (FluidLoss, mg from 2,000 Hour Aging) Copper 10 Lead Solder 30 Brass 10 Steel10 Cast Iron 10 Cast Aluminum 30 Erosion Corrosion of Heat Exchanger, Noleaks 2,000 hours Repassivation of Aluminum by E_(B) < 2.0 GalvanostaticMeasurement ASTM D6208 E_(G) > −0.4.0 ASTM D-2570 Simulated ServiceMaximum Weight Multiple (Higher Performance Specification) Loss, mgembodiments Copper 10 passed. Lead Solder 20 Brass 10 Steel 10 Cast Iron10 Cast Aluminum 20

TABLE 3 ASTM 2809 Test Data Inhibitor EG PDO Conventional Automotive 8 9Carboxylate Automotive 2 8 Phosphated Heavy Duty 10 10 Non PhosphatedHeavy Duty 3 8 Hybrid Heavy Duty 9 10

TABLE 4 Oxidation comparison between PDO and EG inhibited withcommercial inhibitor package @ 2.2%. Test run on D-4340 at 150° C.,without corrosive water and at 50% concentration. Time (days) 0 10 20 3040 50 60 pH PDO-A 11.16 9.31 8.87 8.69 8.41 8.19 7.96 EG-A 10.06 7.676.38 5.68 4.60 4.31 4.07 PDO-B 10.58 9.63 8.89 8.56 8.32 8.18 7.93 EG-B10.67 9.22 8.67 8.32 8.02 7.92 7.74 Total Degradation Acids (ppm) PDO-A0 213 415 607 762 851 1029 EG-A 0 542 1553 1987 3498 4028 4705 PDO-B 0231 372 587 688 833 1053 EG-B 0 342 654 922 1128 1486 1602

Example 2

In these experiments, a solution of 50 percent by volume 1,3-propanediol (PDO) and 50 percent by volume deionized water were tested forcorrosion of various metals used in engine cooling systems over a periodof time. The test method was modified from ASTM test method D-2570 byusing the spaced interval examination procedure detailed in ASTM G-31.The following Table 5 shows the results: TABLE 5 Extended SpacedInterval Simulated Service Test Modified from ASTM D2570 (using ASTM G31spaced interval) Test Method PDO @ 50% in DI Water 190° F. (88° C.).Spaced Interval Corrosion Data Weeks 2 4 6 8 10 Copper 2 2 1 1 2 LeadSolder 3 2 6 6 3 Brass 2 2 3 3 4 Steel 11 12 13 13 13 Cast Iron 13 10 1111 40 Cast Aluminum 22 34 40 40 40Note how the corrosion behaves after 8-10 weeks. The fact that thealuminum corrosion does not increase after 6 weeks gives an indicationthat there is some flash corrosion initially but after that the oxidesprotect the aluminum. Generally, the absolute limit is specified by ASTMD3306 to be 60 mg of aluminum lost after 7 weeks' exposure.

Example 3

The next experiment was corrosion of aluminum services over an extendedperiod of time. The results are set out in Table 6 below. TABLE 6Corrosion of Heat Rejecting Aluminum Surface Modified from ASTM D4340Temperature elevated to 300° F. (149° C.), Time extended from 1 week to30 days 50% PDO 50% (volume) DI Water Before Test 10 Days 20 Days 30Days Weight loss — 0.0 0.0 0.0 mg/cm²/week pH 6.55 5.34 4.60 4.99Conductivity μmhos/cm 0 9 9 14 Comments No damage to No damage to Nodamage to No damage to specimen specimen specimen specimen

Please note that even after running this test for 30 days, there was noapparent corrosion damage to the specimen.

Example 4

This example describes experiments following the ASTM D1384 test method,modified by omitting the corrosive salts and were also made to operateat 150 degrees C. by changing the bath from water to 50% propyleneglycol. The tests were done to test the corrosivity of solutions of PDOin water having amounts of PDO from 55 to 85 percent by weight. We haveidentified the 65 weight percent PDO solution as being the best becauseit offered the best overall protection for the six metals tested.However, the data in Table 7 also shows that solutions containing 55%and 60% PDO in water also achieved very good results because fuel cellsystems are most likely to be manufactured primarily of aluminum andstainless steel. TABLE 7 Percent PDO in water 55 60 65 70 75 80 85Copper 1.2 2.0 1.6 1.7 1.7 1.6 0.6 Lead Solder 123.8 93.5 62.5 60.3 39.263.7 20.3 Brass 2.1 1.7 1.8 2.0 1.7 2.7 1.2 Steel 126.1 86.8 84.6 15.829.2 26.3 1.5 Cast Iron 247.6 186.6 263 255.3 227.1 189.3 −0.7 CastAluminum 8.2 7.0 7.3 16.6 17.3 47.5 26.5 Conductivity Before 0 0 0 0 0 00 Test μmhos/cm Conductivity After 30 22 10 7 4 3 0 Test μmhos/cmSummary of Results

We believe that the results show that these PDO-based coolants can beused for a low conductivity application in fuel cell powered systems,including fuel cell vehicles. PDO is demonstrated to be non-conductiveand manifests corrosion resistant properties to the point of meritingserious consideration. The following are some of the more significantfindings:

-   A coolant with high electrical resistance (low conductivity) has    been developed that is appropriate for use in fuel cell powered    systems, including fuel cell powered vehicles, that generate strong    electrical fields. It has electrical resistivity of more than 250    kohm-cm. Ethylene glycol is too corrosive to be completely    nonconductive.-   The coolant, containing PDO, can be formulated in various    concentrations to achieve freeze points of −40 (° F. or ° C.) or    lower (see freeze point graphs in FIGS. 1 and 2).-   The coolant offers more favorable boiling points in aqueous    solutions than traditional glycol based coolants, as high as 471° F.    (234° C.).-   The thermal conductivity is comparable to glycol-based coolants (in    water).-   The viscosity is comparable to glycol-based coolants (in water).-   The heat capacity is comparable to glycol-based coolants (in water).-   The durability is better than glycol based coolants, offering the    prospect of a closed, lifetime-filled low or no maintenance coolant    system.-   The coolant is compatible with system materials, including aluminum    and elastomers.-   The coolant is less toxic and less palatable than ethylene glycol    and is much less likely to be involved in pet or child poisonings.-   The cost of the coolant over the life of the system is comparable to    existing premium coolants.

The physical property data for PDO and potentially competing coolants,ethylene glycol (EG) and propylene glycol (PG) are shown in Table 8:TABLE 8 Physical Properties PDO EG PG Mol. Wt.  76.1  62.07  76.1Boiling Point, ° F. (° C.) 417.9  387.7  369.3 (214.4) (197.6) (187.4)Flash Point, ° F. (° C.) 265  240  220 (129) (116) (104) SpecificGravity, 20° C.  1.0526   1.115   1.032 Freeze Point, 50% solution, ° F.−21  −36  −28 (° C.) (−29) (−38) (−33) Pour Point, ° F. (° C.) <−75 <−71(<−59) (<−57) Viscosity, cP 20° C.  52  17  49 Specific Heat, 212° F.BTU/lb/F  0.652   0.665   0.704 [kJ/(kg*K)] (2.730) (2.784) (2.948)Thermal Conductivity, 25° C.  0.127   0.147   0.119 BTU/hr-ft- F[W/(m*K)@ 25° C.] (0.220) (0.254) (0.206) Heat of Vaporization 25° C., BTU/lb410  449  379 [kJ/kg @ 25° C.] (954) (1044) (882) Purity  99.7  94.5  99

1. A method for cooling a fuel cell engine, comprising cooling a fuelcell engine with a coolant comprising 1,3-propanediol.
 2. The method ofclaim 1 wherein said coolant is an aqueous solution comprised of between1% to 100% by volume of 1,3-propanediol.
 3. The method of claim 2wherein the solution is comprised of from 40% to 85% by volume of1,3-propanediol.
 4. The method of claim 1 wherein the coolant has afreezing point of less than −40° C.